Stabilisers are well known for centralising a drill string in a hole and when the stabiliser is adjustable to permit a change in direction of a drill string. Stabilisers involve surfaces, often cylindrical in part, either straight or spiraled, that bear against the surface of the hole being drilled. Such surface is of course hard and uneven and causes substantial wear of the part bearing against it. It is known to face the bearing surfaces with a plurality of tiles of tungsten carbide. See, for example, WO-A-99/05391. It is also known to use hard material, such as diamond and WO-A-99/05391 suggests inserts in the facing edges of stabilisers comprising tungsten carbide, polycrystalline diamond and cubic boron nitride.
The problem with polycrystalline diamond, or any hard facing elements, is attaching them to surfaces of tools. The tools themselves are generally made of a steel that has the correct strength and toughness for the applications the tool is intended for. However, even if it was cost-effective to do so, a tool would never be made entirely out of material that was hard enough to withstand the rigours of bearing against the bore surface during drilling operations. However, attaching a layer of material that is strong enough is difficult, particularly on surfaces that are moving in a direction parallel to the surface, with objects striking the surface tending to swipe off elements attached to the surface.
Tungsten carbide can be brazed to steel easily and securely enough, particularly when an entire surface is covered. However, WO-A-95/27588 addresses the problem of holding elements still while they are being brazed, because, as soon as braze melts, it reduces friction substantially between element and substrate and, on curved surfaces such as stabilisers, they tend to fall off. The solution proposed by WO-A-95/27588 is to tack-weld the elements first, so that the subsequent brazing step does not dislodge the elements and they can be accurately positioned.
However, tungsten carbide is not the hardest material available and polycrystalline diamond (PCD), and its construct, thermally stable polycrystalline diamond (TSP), are known to be harder and tougher in these applications. However, most braze materials do not readily wet TSP and so connecting them reliably is difficult. PCD is not always suitable, in any event. PCD generally includes the catalyst that facilitates the binding of the diamond crystals together. Because these tend to have different thermal properties, high temperature applications are frequently ruled out for PCD because differential expansion of catalyst can break up the PCD. Also, PCD tends to be chemically reactive, particularly in contact with steel, and may reduce diamond to graphite, carbon monoxide and carbon dioxide.
WO-A-99/05391 suggests embedding the TSP in pockets at the edge, and this works reasonably well when the direction of impacts is into the pockets. Indeed, PCD and TSP are used frequently on drill bits where precisely this direction of impact applies. However, as mentioned above, on stabilisers, the direction of impacts tends to be transverse, and, in any event, there are not the pockets in which to locate the TSP/PCD elements because they are to cover a large part of the surface of the stabiliser.
It is known to coat TSP and PCD with less hard material so that the elements formed in this way are capable of being connected to a substrate, but it is an expensive process to manufacture composite elements comprising a core of TSP or PCD and a coat of less hard material. EP-A-1212511 and EP-A-1036913 both suggest embedding TSP in a larger body of tungsten carbide. This results in a satisfactory product, but holding a TSP element inside a mould in which a tungsten carbide piece is to be sintered is problematic. EP-A-1212511 suggests providing a coating of electrically conductive material on the PCD so that they can be tack welded and then brazed in place.
GB-A-2323112 discloses a method of breaking out of a cased wellbore using a drilling bit that first cuts through the steel casing and second cuts the geological formation. Composite cutting inserts are disclosed that comprise an annular body of tungsten carbide (or Al2O3, TiC, TiCN, TiN or the like, suitable for milling steel and non-reactive therewith) provided with a chip-breaking edge and a cylindrical body of ultra-hard material such as polycrystalline diamond. However, no method of connection is disclosed between them.
GB-A-2431948 discloses a catalyst-free polycrystalline ultra hard body (TSP) brazed to two separate back and side support elements (tungsten carbide) for use on a drill bit. Two support elements are employed so that they better support the TSP body and so that they can move relative to each other when the components are brazed together to remove voids and unwanted stress concentrations.
It is an object of the present invention to provide a simple construction of wear surface incorporating hard materials such as TSP.